I found this plant in a rooftop garden in Pittsburgh, PA. Any ideas as to what it is?
Gardening & Landscaping Stack Exchange is a question and answer site for gardeners and landscapers. It only takes a minute to sign up.Sign up to join this community
I can't see the leaves, but if the picture was taken at the end of summer/autumn, it's probably Michaelmas Daisy.
Aster Azureus, A native aster. This would make a good roof-top plant because it tolerates very dry soil in full sun. http://www.prairienursery.com/store/native-plants/sky-blue-aster-aster-azureus